What's impolite can differ across cultures; some gestures or phrases may be offensive in one place but not another. Not making eye contact is impolite in some cultures but a sign of respect in others.
Pro Tip 2/3
Subtle Signals
Sometimes a tone of voice or facial expression can make a phrase impolite, even if the words are not rude by themselves. Saying 'thank you' in a sarcastic tone can be impolite.
Pro Tip 3/3
Accidental Rudeness
You can appear impolite without intending to, such as when speaking a new language and misusing phrases or expressions. Using informal language in a formal setting may seem impolite.